LAGRANGE POINT 2 (WHTM) — The Webb Telescope has made a discovery – or maybe the absence of discovery – which brings to mind one of the original Sherlock Holmes stories. In Silver Blaze, Holmes makes reference to “The curious incident of the dog in the night-time.” A Scotland Yard detective points out the dog did nothing in the night-time, to which Holmes responds “That was the curious incident.”
On May 15, the James Webb Space Telescope team announces a study advancing our knowledge of where all the water on Earth came from. Scientists turned the telescope’s NIRSpec (Near-Infrared Spectrograph) instrument on Comet 238P/Read, located in our main asteroid belt. The images confirmed water vapor around the comet – the first time it’s been detected around a comet in the main asteroid belt.
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Comet Read is what’s called a main-belt comet. It’s a relatively new classification – an object that stays in the main asteroid belt but which sometimes displays a halo or coma, and tail like a comet. Until main-belt comets were discovered, the presumption was that comets resided beyond the orbit of Neptune, in the Kuiper Belt and Oort Cloud. There, further from the Sun -way further from the Sun, ice could be preserved.
Then as comets fall into the inner Solar System frozen material vaporizes, giving them their distinctive coma and streaming tail. (This is what makes comets, and not asteroids.) The big question was, could water be preserved in the warmer temperatures of the main asteroid belt, between Mars and Jupiter? The Webb observations proved that it could.
This illustration of Comet 238P/Read shows the main belt comet sublimating—its water ice vaporizing as its orbit approaches the Sun. This is significant, as the sublimation is what distinguishes comets from asteroids, creating their distinctive tail and hazy halo, or coma. (NASA/ESA) 
This image of Comet 238P/Read was captured by the NIRCam (Near-Infrared Camera) instrument on NASA’s James Webb Space Telescope on September 8, 2022. It displays the hazy halo, called the coma, and tail that are characteristic of comets, as opposed to asteroids. The dusty coma and tail result from the vaporization of ices as the Sun warms the main body of the comet. IMAGE: NASA, ESA, CSA, Mike Kelley (UMD) IMAGE PROCESSING: Henry Hsieh (PSI), Alyssa Pagan (STScI) 
This graphic presentation of spectral data highlights a key similarity and difference between observations of Comet 238P/Read by the NIRSpec (Near-Infrared Spectrograph) instrument on NASA’s James Webb Space Telescope in 2022 and observations of Comet 103P/Hartley 2 by NASA’s Deep Impact mission in 2010. Both show a distinct peak in the region of the spectrum associated with water. Finding this in Comet Read was a significant accomplishment for Webb, as it is in a different class of comets than Jupiter-family comets like Hartley 2, and this marks the first time that a gas has been confirmed in such a main belt comet. (ILLUSTRATION: NASA, ESA, CSA, Joseph Olmsted (STScI)
“Our water-soaked world, teeming with life and unique in the universe as far as we know, is something of a mystery – we’re not sure how all this water got here,” said Stefanie Milam, Webb deputy project scientist for planetary science and a co-author on the study reporting the finding. “Understanding the history of water distribution in the solar system will help us to understand other planetary systems, and if they could be on their way to hosting an Earth-like planet,” she added.
“In the past, we’ve seen objects in the main belt with all the characteristics of comets, but only with this precise spectral data from Webb can we say yes, it’s definitely water ice that is creating that effect,” explained astronomer Michael Kelley of the University of Maryland, lead author of the study.
Okay, all well and good, you may say. But what about the dog that did nothing in the night? Well, when Webb sent back its imagery, researchers had a surprise waiting for them; Comet 238P/Read had no detectable carbon dioxide.
Usually, the volatile material in a comet that can be easily vaporized by the Sun’s heat includes about 10 percent carbon dioxide. Why wasn’t the CO2 dog barking? The science team has two possible explanations for the missing carbon dioxide. Possibility one-Comet Read had carbon dioxide when it formed but it’s all boiled off due to warm temperatures in the belt.
“Being in the asteroid belt for a long time could do it – carbon dioxide vaporizes more easily than water ice and could percolate out over billions of years,” Kelley explains.
Possibility two, Comet Read may have formed in a particularly warm pocket of the solar system, where no carbon dioxide was available.
The next step? Check out other main-belt comets, and compare them to Comet Read. Astronomer Heidi Hammel of the Association of Universities for Research in Astronomy (AURA) points out “These objects in the asteroid belt are small and faint, and with Webb we can finally see what is going on with them and draw some conclusions. Do other main belt comets also lack carbon dioxide? Either way, it will be exciting to find out,”
